This application relates to electric lamps and particularly to electric lamps having a light source in the form of a discharge tube mounted within an outer envelope.
Electric lamps of the metal halide type include a light source in the form of a discharge tube containing an inert gas and a metal halide filling. The discharge tube is mounted by a frame structure comprised of straps and metal rods within an outer glass envelope. Illumination of the discharge tube is caused by the striking of an arc between upper and lower main electrodes in the discharge tube. In many of these lamps, in order to start the arc an auxiliary (starting) electrode is utilized. The auxiliary electrode is spaced in close proximity to one of the main electrodes. During starting, current is applied by means of starting resistor between one of the main electrodes and the auxiliary electrode to break down the gas and start an arc. In many of these lamps, after the arc is started, the auxiliary electrode is switched out of the lamp circuit. The mechanism for switching the auxiliary electrode out of the circuit is contained within the glass envelope and is usually activated by thermal means such as a bi-metallic element.
Lamp assemblies using a bi-metallic element to disconnect the starting electrode are classified into two types, the opening type and the shorting type. In the opening type the bi-metallic switch is normally closed so that the auxiliary electrode is connected to the current source through the starting resistor. As the lamp warms up the bi-metallic switch element becomes heated, by the heat generated by the discharge tube, the element curves and opens the circuit to the starting electrode. In the shorting type arrangement, the starting resistor is usually disposed in the main circuit and the bi-metallic element, when heated, shorts out the starting resistor to remove it and the auxiliary electrode from the circuit by placing them at the same potential as the main electrodes. Both of these approaches have been, in the past, less than completely satisfactory.
The shorting type of bi-metallic switch generally results in a simplified frame and switch arrangement. However, such designs are often less reliable than the opening type as the proper shorting out of the staring circuit can not be assured as the components age and become oxidized. If the starting circuit is not shorted out catastrophic failure of the discharge tube can result. On the other hand, the opening type of bi-metallic switch arrangement is more reliable and less prone to catastrophic failure. However, these arrangements have resulted in a complex switch and frame structure which does not utilize the switching assembly as part of the frame. Complex frame and switch arrangements make it difficult to utilize automated assembly processes to save on manufacturing costs.
The present invention is directed towards providing a lamp that utilizes the more reliable opening type switch assembly while, at the same time, providing a simple frame arrangement which is suitable for automatic assembly. Furthermore, the present design facilitates the use of single size frame components across different size lamps, thus further minimizing assembly costs.